A wireless communication system using a radio wave in a high frequency band such as a millimeter wave band uses a beam antenna having a high gain and sharp directivity in order to compensate for high propagation loss. Generally, a wireless communication system performs beamforming to control a beam direction toward an opposite station as a communication partner, and tracks the beam direction to follow the direction to perform communication. When beamforming is performed, it is necessary to sequentially search for an optimum beam direction between a local station and an opposite station during communication.
A conventional search procedure described in Non-Patent Document 1 will be described with reference to FIG. 9 exemplifying a wireless communication system 7 including three wireless stations shown in FIG. 8. In this example, each of wireless stations 70-1 to 70-3 includes a variable beam antenna and a transceiver at a 60 GHz band. Therefore, the wireless stations 70-1 to 70-3 can vary a sharp beam having a high gain within a range of directions 1 to 4. The variable beam antenna can also form a quasi-omnidirectional beam having a low but nearly uniform gain within the range of directions 1 to 4. Note that the wireless communication system 7 has a star type network topology in which the wireless station 70-1 is a master station, and the wireless stations 70-2 and 70-3 are slave stations.
Part (a) of FIG. 9 is a time chart of signal transmission in the wireless communication system 7. A signal to be transmitted has a cyclic frame structure, and each frame includes a beam direction determination section and a main signal communication section. The beam direction determination section is a section during which a beam direction of a variable beam antenna of each wireless station is determined. The main signal communication section is a section during which communication with a main signal is performed. The main signal is a signal including main information to be transmitted to and from a user of an opposite communication station.
A processing procedure in the beam direction determination section is shown in part (b) of FIG. 9. First, the wireless station 70-1 sequentially changes a direction from direction 1 to direction 4 while the wireless station 70-1 transmits a direction search signal in each beam direction. On the other hand, the wireless station 70-2 defines beam directivity thereof as a quasi-omnidirectional beam and measures a reception condition of the direction search signal transmitted by the wireless station 70-1. Next, the wireless station 70-2 transmits a direction search signal with a beam direction sequentially directed in each of directions 1 to 4. The wireless station 70-1 sets the directivity thereof as a quasi-omnidirectional beam and measures a reception condition of a received direction search signal for each beam direction. At this time, the direction search signal transmitted by the wireless station 70-2 includes transmission direction information of the wireless station 70-1 when the reception condition concerning the direction search signal from the wireless station 70-1 was the best at the wireless station 70-2. The wireless station 70-1 determines a beam direction used for communication with the wireless station 70-2 according to the received direction search signal. The wireless station 70-2 transmits a direction search signal in any one of directions 1 to 4. Thereafter, the wireless station 70-1 notifies the wireless station 70-2 of a transmission direction in which a reception condition is the best among direction search signals from the wireless station 70-2. The wireless station 70-2 determines a beam direction used for communication with the wireless station 70-1 according to this notification. A similar procedure is also performed between the wireless stations 70-1 and 70-3. Therefore, beam directions for all the opposite stations are determined. Since the beam direction determination section appears every constant frame cycle, the beam direction is updated sequentially.